Communication cables that are strung between poles or those buried in the ground are subject to abuse such as, for example, attack by rodents, mechanical abrasion and crushing. Attacks by gophers and other burrowing rodents on buried cable and by squirrels on aerial cable have been a continuing concern. Testing with gophers has evolved into a normal criteria in the industry for evaluating cables. Gophers, for example, have been shown to exert biting pressure as high as 18,000 psi. Cables having an outside diameter below a critical size of about three-quarters of an inch (0.75xe2x80x3) in diameter are more susceptible to being damaged than larger cables because the animals can bite directly down on them, encompassing the entire cable with their jaws. For larger size cables, generally only a scraping or raking action takes place. In fact, on cables exceeding about two inches (2xe2x80x3) in diameter, gopher attacks are rarely observed.
It has been found that with limited exceptions, the only way to protect directly exposed cables from rodent attack is to wrap them in a sufficiently hard and tough metallic shield, or an armor. A longitudinally applied shield, if otherwise suitable, would be economically preferable from a manufacturing standpoint. For cables below the critical size, the use of a corrugated shield having a longitudinally overlapped seam generally has provided sufficient protection. Typically, the corrugation pattern disposed on a corrugated shield is in the configuration of substantially uniformly sized and substantially uniformly spaced laterally raised portions and laterally lower portions which, in profile, resemble a sine wave. These corrugated armors, while they provide protection to the cable from rodent attacks and flexibility needed for easy storage and installation, lack tensile stiffness essential to the durability of the cable.
Lately, optical fiber cables have become predominant in the communications cable market. They, when buried in the ground, are subject to rodent attack and when strung between poles are subject to damage by squirrels. Another prior art optical fiber cable sheath system which offers rodent protection comprises two helically wrapped, non-corrugated stainless steel shielding tapes enclosed in a plastic jacket. However, this arrangement has several shortcomings. It is expensive to manufacture because of low line speeds, the complex machinery required to wrap the tapes helically about a core, and the separate steps required to accomplish taping and jacketing.
Yet another prior art optical fiber cable, which offers rodent protection, comprises a corrugated armor shield wrapped around an optical fiber, or fiber bundle, with additional longitudinally extending tensile stiffness members disposed outside the armor where the tensile stiffness members extend the length of the cable. Similar to the armor discussed above, the corrugation pattern of the prior art comprises a substantially symmetric, sinusoidal-like wave pattern. The armor is typically metallic and difficult for a rodent to penetrate, however; while the corrugation pattern provides flexibility, corrugations significantly decrease the tensile stiffness of the armor at low strains. As optical fiber cables typically operate only at low tensile strains ( less than 1%), the corrugated armor alone provides insufficient tensile stiffness to the cable sheath. As such, tensile stiffness is added to the cable with separate components. These components may be, for example, a pair of tensile stiffness members disposed opposite each other, one on either side of the core, many small members distributed in a helix around the outside of the core, a single tensile member located along the longitudinal axis of the cable, or any combination of such members. All of these tensile members, central, distributed and pairs, typically extend the length of the cable.
The prior art configurations described above have several shortcomings. These cables can be more expensive to produce since the functions of structural armor protection and of tensile stiffness are provided by separate components. Furthermore, the cables having diametrically opposed stiffness member systems are more difficult to store and install because such cables resist bending in all but one plane. These cables are also large in diameter, as compared to the core housed within. For cables with many distributed tensile members, more complex manufacturing facilities and more intricate fasteners for cable termination can be required.
As such, it can be seen that a cable having an armor capable of providing tensile stiffness and resistance to penetration by rodents and damage by other mechanical hazards that is also flexible and cost effective to manufacture is needed. Thus, a heretofore unaddressed need exists in the industry to overcome the aforementioned deficiencies and inadequacies.
The present invention is a protective cable armor having significant tensile stiffness at low strains and providing structural protection from invasion by foreign objects. The armor comprises a substantially planar sheet member having a length and a width and an intermittent corrugation pattern disposed therein. The intermittent corrugation pattern comprises a series of alternating grooved sections and land sections (non-corrugated portions) adjacent to one another, each with a defined width such that a consistent pattern of grooved and land sections is repeated along the length of the sheet member. The sheet member then can be disposed in a substantially tubular form.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.